In an image forming process using an electrophotographic method, an electrostatic latent image is formed on an inorganic photoreceptor such as selenium, selenium alloy, cadmium sulfide, and amorphous silicon or an organic photoreceptor using a charge generating agent and a charge transporting agent, and the formed image is developed by a toner, transferred to a paper or a plastic film, and fixed to obtain a visible image.
As for the photoreceptors, depending on the composition, there are those positively charged and those negatively charged. In the case of forming printing parts as an electrostatic latent image by exposure, the image is developed by a toner having an opposite sign electrical charge. Meanwhile, in the case of reversely developing printing parts by removing the electricity of the printing parts, the image is developed by a toner having a same sign electrical charge. A toner is constituted of binder resin, a coloring agent, and other additives, and a charge control agent is usually used therein to provide desired frictional charge characteristics (charge speed, charge level, charge stability, and the like), temporal stability, and environmental stability. This charge control agent largely affects the characteristics of the toner.
Further, in the case of color toners, a light-colored charge control agent, desirably colorless charge control agent, which does not affect the hue is needed. Examples of these light-colored or colorless charge control agents include metal complex salt compounds of hydroxybenzoic acid derivatives (see Patent Literatures 1 to 3), aromatic dicarboxylic acid metal salt compounds (see Patent Literature 4), metal complex salt compounds of anthranilic acid derivatives (see Patent Literatures 5 and 6), organic boron compounds (see Patent literatures 7 and 8), biphenol compounds (see Patent Literature 9), calix[n] arene compounds (see Patent Literatures 10 to 15), and cyclic phenol sulfides (see patent Literatures 16 to 18) for a negatively charged toner, and quaternary ammonium salt compounds (see Patent Literatures 19 to 21) for a positively charged toner.